Conventionally, semiconductor memories used in electronic devices such as microcomputers have mainly employed dynamic random access memories (DRAMs) from the viewpoint of operating speed and the degree of device integration. It is difficult, however, for DRAMs to accommodate the recent demands for lower energy consumption and mobility because DRAMs consume energy for memory storage purposes and the stored data is lost once power supply is turned off. In order to address such demands, a novel memory is required that is nonvolatile as well as fast, highly integrated, and energy-saving.
Magnetoresistive random access memories (MRAMs) are now gaining attention as a next-generation memory with the nonvolatile property, in addition to being capable of achieving operating speeds and levels of integration comparable to those of DRAMs. The MRAM stores information in terms of the direction of magnetization of a ferromagnet. The relative magnetization configuration of the information stored in the MRAM is electrically sensed utilizing the giant magnetoresistance effect in a spin valve element, or the tunneling magnetoresistance (TMR) effect in a magnetic tunnel junction (MTJ). Since MRAMs utilize a ferromagnet, they can hold information in a nonvolatile manner without consuming energy.
FIG. 17 shows a typical cell configuration of an MRAM utilizing a MTJ. As shown in FIG. 17(A), the MRAM comprises a 1-bit memory cell consisting of one MTJ and one metal oxide semiconductor (MOS) transistor. The gate of the MOS transistor is connected to a wordline for sensing, the source is grounded, and the drain is connected to one end of the MTJ. The other end of the MTJ is connected to a bitline.
As shown in FIG. 17(B), the MTJ has a tunnel junction structure consisting of two ferromagnetic electrodes separated by a thin insulating film. The MTJ provides the TMR effect in which tunnel resistance varies depending on the relative magnetization configuration of the two ferromagnetic electrodes. The rate of change of TMR between the case where the two ferromagnetic electrodes carry parallel magnetization and the case where they carry antiparallel magnetization is referred to as the TMR ratio, which is used for the evaluation of the TMR effect.
In the MRAM, information is stored in terms of the configuration of magnetization of the MTJ. Specifically, the relative magnetization configuration of the two ferromagnetic electrodes is rendered either parallel or antiparallel using a composed magnetic field formed by magnetic fields induced by currents that are caused to flow through the bitline and a wordline for writing (not shown) disposed perpendicular to the bitline.
When sensing information stored in a particular cell, a voltage is applied to a specific wordline for sensing connected to the cell so as to bring the MOS transistor into conduction, so that a current for sensing (to be hereafter referred to as a “drive current”) flows through the MTJ via a specific bitline connected to the cell. A voltage dropped across the MTJ due to the TMR effect is then detected as an output voltage to sense the stored information.